1. Field of the Invention
The invention relates generally to a battery charger. More particularly, the invention relates to a battery charge controller having an adjustable termination current.
2. Description of Related Technology
Batteries may be generally classified as either rechargeable or non-rechargeable. For instance, lithium-ion batteries, lead-acid batteries, and nickel-cadmium batteries are rechargeable batteries. As is widely known, rechargeable batteries are capable of storing a finite amount of charge (i.e., energy). Thus, as electrical energy is conveyed to a rechargeable battery during the charging process, a charge termination point is reached at which the battery cannot store any additional energy. Supplying electrical energy to a rechargeable battery that has reached the charge termination point can severely reduce the operating life of the battery because the additional electrical energy provided to the battery is dissipated as heat within the battery.
Because overcharging a rechargeable battery can significantly reduce the operating life of the battery, battery chargers that are adapted to discontinue the charging of the battery at the charge termination point are highly advantageous. In fact, some battery chargers have a charge controller that determines the charge termination point to discontinue the charging of the battery. One such type of charge controller monitors battery charging current to determine the input impedance of the battery being charged. Charge controllers that determine battery impedance to control the charging process are based on the fact that battery impedance increases as the charge level of the battery approaches the charge termination point. Additionally, these types of battery chargers often include user adjustments for adjusting an initial maximum charging current and have a termination/minimum charging current that is fixed at a constant value. For example, in charging a lithium-ion battery the initial current is typically varied between 500 mA to 1 A and the termination current is set at a fixed value in the range of 50 mA to 100 mA. Thus, when the charging current detected by the charge controller reaches 50 mA to 100 mA, the charge controller controls the battery charger to terminate the charging current so that no additional energy is provided to the battery.
In applications requiring high-capacity rechargeable batteries, such as notebook computers, the initial current and termination current must also be set. In some high capacity applications a plurality of rechargeable batteries are configured in parallel so that the initial charging current is determined based on the initial charging current required per battery multiplied by the number of batteries that are coupled in parallel. In conventional battery chargers the initial current value can be adjusted based on the total capacity of the rechargeable battery; however, the termination current is generally fixed at a constant value regardless of the capacity of the rechargeable battery. As a result, the rechargeable battery may be undercharged or overcharged and, in the case of overcharging, the life span of the battery can be significantly reduced.
FIG. 1 is a graph illustrating the relationship between the charging current and charging time for conventional charge control techniques. Curves (1), (2), and (3) correspond to rechargeable batteries having different capacities. For example, curves (1), (2), and (3) may represent the charging characteristic for a single battery cell, two battery cells, and three battery cells, respectively. Accordingly, each of the curves (1), (2), and (3) shows an initial charging current that is proportional to the total battery capacity. Namely, curve (1) represents the smallest capacity and has the smallest initial charging current Imax1, curve (2) represents the next largest battery capacity and has a proportionally larger initial charging current Imax2, and curve (3), which represents the largest battery capacity, has the largest initial charging current Imax3. As shown in FIG. 1, each of the curves (1), (2), and (3) has the same charge termination current Imin.
Thus, FIG. 1 illustrates a conventional charge termination control technique that is based on using the fixed charge termination current Imin regardless of the charge capacity of the battery being charged. As can be seen in FIG. 1, the fixed termination current Imin causes the charging time to vary proportionally with the battery capacity. Additionally, the fixed termination current can result in overcharging of the battery depending on the capacity of the battery being charged because the per cell impedance for a given termination current increases as the capacity (i.e., the number of cells) increases. Namely, as the termination current on a per cell basis decreases, the final impedance and the charge level increase on a per cell basis.
For instance, if the termination current is fixed at 100 mA, which may be appropriate for a single battery cell, and the battery being charged consists of four parallel connected battery cells then each battery cell receives 25 mA at the termination point, which is indicative of a high impedance and charge level that could result in a damaging overcharged condition. In contrast, if the charge termination current of 400 mA, which may be appropriate for four cells, is used to terminate the charging of a single cell, the battery cell receives 400 mA at the termination point, which is indicative of a low impedance and charge level that could result in an undercharged condition.